1. Field of the Invention
This invention relates generally to a method for membrane-electrode-assembly (MEA) break-in and voltage recovery and, more particularly, to a method for MEA break-in and voltage recovery that includes increasing a fuel cell stack temperature in a stepwise manner from approximately room temperature to a temperature that is consistent with high fuel cell stack load operation, maintaining constant anode and cathode reactant flows for each temperature step and cycling fuel cell stack current density over a number of cycles to provide an efficient and fast break-in for the MEAs in the fuel cell stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is renewable and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is catalytically split in an oxidation half-cell reaction in the anode catalyst layer to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell type for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely dispersed catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane where it forms the anode and cathode catalytic layers. The combination of the anode catalytic layer, the cathode catalytic layer and the membrane define a membrane electrode assembly (MEA). MEAs require adequate fuel and oxidant supply and also humidification for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack typically includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The MEA within a fuel cell needs to have sufficient water content so that the ionic resistance across the membrane and anode and cathode catalytic layers is low enough to effectively conduct protons. Humidification for the membrane and the ionomer in the catalytic layers may come from the stack water by-product or external humidification. The MEAs in a newly built fuel cell stack are dry, i.e., they essentially lack ionic conductivity.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will typically include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow. Water in the cathode exhaust gas at one side of the water transfer elements, such as membranes, is absorbed by the water transfer elements and transferred to the cathode air stream at the other side of the water transfer elements.
Both break-in or conditioning and voltage recovery are required for the MEAs in a newly fabricated fuel cell stack to obtain optimal performance during initial operation of the stack. There are three main functions of MEA break-in and voltage recovery: humidification, removal of residual solvents and other impurities from MEA manufacturing and removal of anions from the catalyst to activate reaction sites. The current state of the art procedures for break-in and voltage recovery of the MEAs require from 1 to over 15 hours of fuel cell operation targeting different levels of resulting functionality. Thus, there is a need in the art to provide a method of break-in and voltage recovery of the MEAs in a short period of time that is still capable of achieving the three main functions and that also provides the targeted performance level.